The present invention relates in general to heated tip fuel injectors, and, in particular, to a method of using heated tip fuel injectors to reduce hydrocarbon (HC) emissions in internal combustion engines.
Today's standards for low and ultra-low emissions vehicles require increased research and development in unburned HC emissions, particularly in operations such as engine cold starts. In this operating mode, the initial compression strokes ordinarily take place with cold intake valves, and cold port and cylinder walls. Consequently, the fuel evaporation rate is slow even though the overall air/fuel (A/F) mixture is within ignitable limits. These effects become more severe if the ambient temperature drops below 0.degree. C. Also, conventional three-way catalytic converters used for exhaust gas after treatment are ineffective in oxidizing unburned HCs until heated to their "light-off" temperature by the heat transfer from exhaust gases.
The automotive industry is making a strong effort to decrease the catalyst "light-off" time and, thus, decrease HC emissions. A more reasonable approach is to reduce the cold enrichment with a more complete atomized spray because unburned fuel causes these emissions. Several studies reported using technologies such as air-assist injectors, preheated intake ports, or engine blocks to make just such improvements. However, the automotive industry has adopted few of these technologies because of their increased engine complexity or insufficient level of spray atomization.
The level of fuel atomization is sufficient if the spray droplets are small enough to be entrained by the intake air flow. The fuel then can be transported into the cylinder without depositing on the intake port or cylinder wall. An estimated 20 .mu.m-droplet size is required to avoid spray impingement and follow the air flow, assuming a common intake port geometry and low air speeds.
The present invention enhances spray atomization, especially during cold starts, by heating fuel inside the injector. A high percentage of the fuel immediately vaporizes when the liquid exits the orifice (flash boiling). The energy released in flash boiling breaks up the liquid stream, creating a vapor mixture with droplets smaller than 25 .mu.m.
Even though the advantages of fuel vaporization by heating are well known, the automotive industry has not adopted several concepts because they are considered impractical. Nevertheless, by heating the fuel inside the fuel injector, the heated tip injector has several advantages. The heater is in direct contact with the fuel, which promotes faster heating. In addition, the heater can be turned off when not needed, allowing the heated tip injector to function as a normal port fuel injector with well-defined targeting.